Using a first-principles high-throughput screening approach, we systematically investigate the two-dimensional (2D) pentagonal $P\overline{4}{2}_{1}m {\mathrm{XY}}_2$ prototype family (where X and Y are main-group elements from Group IA to VIIA and with $\mathrm{Z}=2$ formula units per primitive cell) and identify 37 previously unknown stable phases exhibiting excellent thermodynamic, elastic, and dynamic stability. Analysis of valence electron count and bonding reveals that 2D $P\overline{4}{2}_{1}m {\mathrm{XY}}_2$ phases with 10, 12, and 14 valence electrons (VEs) per chemical formula are semiconducting, whereas those with 11 and 13 VEs exhibit metallic or ferromagnetic behavior. This behavior can be rationalized using a crystal-orbital approach. Notably, 2D $P\overline{4}{2}_{1}m {\mathrm{SbB}}_2$ (11 VEs), ${\mathrm{AsB}}_2$ (11 VEs), and ${\mathrm{LiO}}_2$ (12 VEs) are predicted to be ferromagnetic semiconductors with Heyd-Scuseria-Ernzerhof band gaps of 1.06, 1.19, and 2.75 eV, respectively. These 2D pentagonal $P\overline{4}{2}_{1}m {\mathrm{XY}}_2$ phases are promising for flexible electronics due to their low in-plane Young's modulus (6–100 N/m) and large critical strain (18%–40%). Moreover, exceptional out-of-plane piezoelectricity (with coefficients $d_{36}$ ranging between 18 and 60 pm/V) is observed in 2D $P\overline{4}{2}_{1}m {\mathrm{SnBi}}_2, {\mathrm{GeBi}}_2, {\mathrm{TeGe}}_2$, and ${\mathrm{PbBi}}_2$ due to their low shear modulus and large piezoelectric stress tensors. This work not only rationalizes the structure-electronic property correlation of the 2D pentagonal $P\overline{4}{2}_{1}m {\mathrm{XY}}_2$ prototype, but also provides valuable guidance for designing multifunctional 2D materials with tunable electronic, mechanical, and piezoelectric properties.

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• Electronic structure
• Piezoelectricity
• Poisson ratio
• 2-dimensional systems
• First-principles calculations
• High-throughput calculations